Universal sublinear population growth density dependence unrelated to resource limitation

Population growth slows as population density increases, and the form of this dependence can sensitively influence species coexistence, conservation, and resource management. Yet the processes shaping density-dependent growth--and whether it exhibits general features across ecological systems--remain unclear. Here, we show that the decline of growth rate with density is consistently sublinear across thousands of distinct growth conditions and bacterial and eukaryotic taxa; and that sublinearity likely results from density-dependent growth limiters which are unrelated to nutrient and energy resources. High-resolution growth experiments with Escherichia coli reveal a two-phase density dependence of growth rate, with an early sublinear regime transitioning to superlinear decline only near saturation. We extend the Monod equation to incorporate a resource-independent growth inhibition, reproducing the crossover and unifying observed patterns. Our analyses identify a universal dual-regime form of density-dependent growth and show that growth inhibition emerges well before resource depletion. Our findings revise classical views of population saturation, revealing non-resource-related inhibition mechanisms as fundamental to shaping population growth dynamics across ecosystems and the important phenomena those dynamics influence.